...
Personally, I've NEVER had a problem with the concept of RMT. Maybe it's because I've always been involved in "collector" markets IRL starting with Baseball cards as a kid, moving to coins during teenage years, into Magic later on, and now work professionally in the coin industry (and am currently selling off Magic cards on ebay).
I think M:tG may be the closest of those in regards to gaming and RMT. As a comparison, you have the initial cost of entry (buying packs for cards, paying your monthly fee for an MMOG), and then you have the option of trading with other players for "resources" within the context of the game. Also as a comparison, as mentioned above you have people who run extra accounts to gain an advantage over those who run single accounts, the same holds true for the person able to buy boxes of cards more than others.
Then you get into the secondary market, the in-game "trading". Sometimes it's for "in-game resources" (card-for-card in M:tG, ingame item for ingame item or in-game currency for MMOGs), other times the trade may be for the "ingame item" in exchange for real money (RMT). With M:tG, it's simply been a part of the game since day one and as is probably well known, certain cards can net $1,000+ in the RMT market. Between my experience in the M:tG realm and my own personal free-market leanings, I'm not anti-RMT in online games.
All that being said and to swing it back to the initial issue of being spammed ingame, I also believe customers/players have the right to have a game experience free from constant RMT spamming (as well as ingame trade spamming which should be relegated to Trade Channels) and that steps should be taken to make sure that such advertising is kept in the correct channel, or if expressly prohibited (the advertisement, not necessarily the actual act of RMT), then removed from the game.
I don't think there's any conspiracy between the game companies and the gold sellers to allow the spammers to continue to spam. I think the more likely explanation, ESPECIALLY with the state of UO currently, is simply that the spammers are SO ABSURDLY LOW on the priority list for both the devs (of whom we ask EVERYTHING) and the GM's (of whom there are few left and have to deal with every paged problem even when the "problem" doesn't require their assistance) that they simply get lost in the shuffle. Spamming isn't a game-creaking bug or any form of direct player-to-player harassment, so it's relegated further down the list.